1. Field of the Invention
The present invention relates to a Faraday rotator, an opto-isolator, and a laser processing apparatus. In particular, the present invention relates to a Faraday rotator that can be suitably used when a polarization-independent opto-isolator is used, an opto-isolator using the Faraday rotator, and a laser processing apparatus including the opto-isolator.
2. Description of the Related Art
As shown in FIG. 6, a conventional laser processing apparatus includes an opto-isolator 111 to stabilize a light source and prevent breakage by blocking optical feedback caused by reflection. The opto-isolator 111 includes, in sequence, a first polarizer 102, a Faraday rotator 103, a wave plate 104, and a second polarizer 105 in a traveling direction of a laser beam FB.
Here, the conventional Faraday rotator 103 includes an element causing the Faraday effect and a magnet (not shown). A crystal causing the Faraday effect, such as TiO2 (rutile), CaCO3 (calcite), α-BBO, or YVO4, and the like are used as the element causing the Faraday effect. The magnet is disposed such that a line of magnetic force of the magnet is generated in parallel with an optical path of the laser beam FB.
Patent Literature 1: Japanese Patent Laid-open Publication No. Heisei 6-34926
However, as shown in FIG. 7, when a high-output laser beam FB of 100 W or more is transmitted through the Faraday rotator 103, a problem occurs in that the temperature of the Faraday rotator 103 rises, and polarization rotation angle changes. When the polarization rotation angle changes, transmittance and extinction ratio of the opto-isolator 111 become unstable. This problem becomes particularly serious when a high-output laser beam FB of 500 W or more is transmitted through the Faraday rotator 103.
The ambient temperature in which the Faraday rotator 103, the opto-isolator 111, and the laser processing apparatus are used is ordinarily 0° C. to 50° C. The temperature of the Faraday rotator 103 also changes as a result of change in the ambient temperature. Therefore, even when a low-output laser beam FB is transmitted through the Faraday rotator 103, a problem occurs in that the polarization rotation angle changes, and the transmittance and the extinction ratio of the opto-isolator 111 become unstable.